Evaporation source, apparatus and method for the preparation of organic El device

ABSTRACT

An evaporation source includes an insulating container adapted to receive a volume of source material therein and a heater closely disposed around the container for heating and evaporating the source material into a vapor. The effective contact area of the container in contact with the source material is correlated to the volume of source material. The evaporation source is useful in the preparation of organic EL devices.

This invention relates to an apparatus and method for preparing anorganic electroluminescent (EL) device, and more particularly, to anapparatus and method for preparing an organic EL device using anevaporation process of heating and evaporating an organic sourcematerial, thereby depositing the material on a selected region of asubstrate to form a thin film thereon. Specifically, it relates to anevaporation source for use in the evaporation process.

BACKGROUND OF THE INVENTION

Vacuum evaporation is well known as one of basic thin-film formingprocesses. In the vacuum evaporation process, an evaporation source anda substrate are placed in a vacuum chamber, and a source material isevaporated to deposit a thin film on the substrate. A variety ofevaporation sources are known. One typical process is a resistanceheating evaporation process of conducting electric current across ametal container or boat having a relatively high electric resistance togenerate heat with which a source material is evaporated, as describedin Appl. Phys. Lett., 68 (16), Apr. 15, 1996, for example. Also known isan electron beam/laser beam evaporation process of directly irradiatingelectron beams or laser beams to a source material for evaporating thematerial with the beam energy. Of these, the resistance heatingevaporation process is widely used in the art because the depositionapparatus is of simple construction so that thin films of quality can beformed at a low cost.

In the resistance heating evaporation process, a metal material having ahigh melting point such as tungsten, tantalum or molybdenum is workedinto a thin plate having a high electric resistance, from which acontainer or boat is made. A source material is placed in the container,which is disposed in a (vacuum) chamber. Direct current is conductedacross the container to generate heat, with which the source material isevaporated to feed a source material gas. A part of the dispersing gasdeposits on the substrate to form a thin film. As the source material tobe evaporated, any of materials having a relatively high vapor pressuremay be used although the material that is chemically reactive with thecontainer should be avoided.

Recently, active research works have been made on organic EL devices. Asa basic configuration, the organic EL device includes a hole injectingelectrode, a thin film formed thereon by depositing a hole transportingmaterial such as triphenyldiamine (TPD), a light emitting layerdeposited thereon of a fluorescent material such as an aluminumquinolinol complex (Alq3), and a metal electrode or electron injectingelectrode formed thereon from a metal having a low work function such asmagnesium. Such organic EL devices are attractive in that they canachieve a very high luminance ranging from several 100 to several 10,000cd/m² with a drive voltage of approximately 10 volts.

In the prior art method of manufacturing organic EL device-appliedproducts, functional thin films of organic materials are formed usingthe evaporation process. It is crucial for such commercial mass-scalemanufacture to increase the productivity and to reduce the percentrejection. However, manufacturing apparatus using prior art evaporationdevices are difficult to achieve mass production and to manufactureproducts of uniformity and hence, high quality because of a low filmdeposition rate and non-uniformity in thickness and composition oforganic layers during the mass-scale manufacturing process. When afunctional thin film such as an electron injecting electrode isdeposited on the organic layer, the organic layer can be damaged orinversely, the electron injecting electrode itself be contaminated withimpurities or oxidized. These lead to defectives such as non-uniformluminance, dot defects, and current leakage as well as qualityvariances.

The evaporation boat is easy to control the rate of evaporation sincedirect resistance heating is possible. The boat, however, canaccommodate therein only a small amount of a source material, lacking apractical utility from the industrial aspect.

On the other hand, a cell type evaporation source can contain a largeramount of source material, but is low in thermal response because ofindirect heating. As a consequence, it is difficult to control the rateof evaporation. The percent utilization of the source material becomeslow when the rate of evaporation is set constant. This makes itdifficult to reduce the cost of products particularly when an expensiveorganic material is used. Also, in the case of evaporation at arelatively low temperature from the cell type evaporation source as inthe deposition of organic layers in organic EL devices, the thermalresponse is further exacerbated because of poor radiating efficiency.

In particular, light emitting layers of organic EL devices are oftenformed by doping a host material with a minor amount of fluorescentmaterial so as to adjust to the desired luminous characteristics. Even aslight shift in the amount of host material or dopant in the mixed layercan jeopardize the luminous characteristics. For these and otherreasons, prior art evaporation equipment are difficult to achieveuniformity of products or produce EL devices of high quality, especiallyin the mass-scale manufacture process.

SUMMARY OF THE INVENTION

An object of the invention is to provide an evaporation source for usein the preparation of an organic EL device which is capable ofcontaining a large amount of source material, enables stable evaporationover a long period of time, enables to adjust and maintain uniform thethickness and composition of a thin film, and allows for evaporation atrelatively low temperatures or on a substrate with a relatively largesurface area.

Another object of the invention is to provide an apparatus and methodfor the preparation of an organic EL device, using the evaporationsource.

A further object of the invention is to provide an evaporation source,apparatus and method for the preparation of an organic EL device whichcan control at high precision the mixing ratio or doping amount inmulti-source evaporation.

In a first aspect, the invention provides an evaporation source for usein the preparation of organic electroluminescent devices, comprising acontainer of an insulator having a volume of source material receivedtherein, and a heater closely surrounding the container for heating andevaporating the source material into a vapor. The container includes aheating zone which is directly heated by the heater and which is incontact with the source material over an effective contact area. Theeffective contact area which is equal to S cm² and the volume of sourcematerial which is equal to V cm³ are controlled to meet V/S≦1 cm.

Several preferred embodiments are given below. (1-1) The container has abottom and a side wall which together define the heating zone. (1-2) Thecontainer has a bottom and a side wall which together define the heatingzone, the container further has a raised portion extending from thebottom, and the heating zone is also associated with the raised portion.(1-3) The container defines an opening over the heating zone, the vaporof the source material scatters through the opening, a vapor density m₀appears at a vertical distance L₀ from the center of the opening, avapor density m appears at a position spaced a distance L from thecenter of the opening at an angle θ, and the value of n obtained byapproximating the vapor density m to be m=m₀·(L₀/L)²·cos^(n)θ is notgreater than 6. (1-4) The heater is capable of evaporating the sourcematerial at a maximum evaporation rate of at least 150 μg/sec. (1-5) Themaximum volume of the source material is at least 5 cm³. (1-6) Thesource material is a sublimable material which is utilized at anefficiency of at least 85%. (1-7) The insulator of the container has athermal conductivity of at least 50 W/m·k. (1-8) The heater issurrounded by a layer of an insulator having a thermal conductivity ofat least 50 W/m·k. (1-9) The insulator of the container or the insulatorof the surrounding layer or both are pyrolytic boron nitride, and theheater comprises carbon.

In a second aspect, the invention provides an apparatus for preparing anorganic electroluminescent device, comprising the improved evaporationsource, a substrate on which the organic electroluminescent device is tobe formed, a means for detecting the rate of evaporation of the sourcematerial on the substrate, and a means for controlling the evaporationsource in accordance with information from the detecting means.

Several preferred embodiments are given below. (2-1) The control meanscontrols so as to keep the evaporation rate constant. (2-2) The controlmeans controls the temperature of the evaporation source. (2-3) Thecontrol means controls the temperature of the evaporation source andthen controls so as to keep the evaporation rate constant. (2-4) Thesource material is an organic material which evaporates at a temperatureof up to 800° C.

In a third aspect, the invention provides a method for preparing anorganic electroluminescent device on a substrate, using the improvedevaporation source. The method involves the steps of actuating theheater for heating and evaporating the source material, detecting therate of evaporation of the source material on the substrate to acquireinformation, and controlling the evaporation source in accordance withthe information, thereby depositing the source material on thesubstrate.

Several preferred embodiments are given below. (3-1) The controllingstep is to keep the evaporation rate constant. (3-2) The controllingstep is to control the temperature of the evaporation source. (3-3) Thecontrolling step is to control the electric current or power applied tothe heater of the evaporation source. (3-4) The controlling stepincludes controlling the temperature of the evaporation source and thencontrolling so as to keep the evaporation rate constant. (3-5) Thesource material is an organic material which evaporates at a temperatureof up to 800° C.

BRIEF DESCRIPTION OF THE INVENTION

These and other objects, features and advantages of the invention willbe better understood by reading the following description, taken inconjunction with the accompanying drawings.

FIGS. 1A and 1B are plan and cross-sectional views of an evaporationsource according to one embodiment of the invention.

FIGS. 2A and 2B are plan and cross-sectional views of an evaporationsource according to another embodiment of the invention.

FIG. 3 is a schematic view of one exemplary arrangement of the apparatusof the invention, showing the evaporation source relative to thesubstrate.

FIG. 4 is a graph showing how to control with time the temperature of anevaporation source and the evaporation rate in a first example of theinvention.

FIG. 5 is a graph showing how to control with time the temperature of anevaporation source and the evaporation rate in a second example of theinvention.

FIG. 6 is a graph showing how to control with time the temperature of anevaporation source and the evaporation rate in a third example of theinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The evaporation source for use in the preparation of organic EL devicesaccording to the invention includes an insulating container having avolume of source material received therein, and a heater closelysurrounding the container for heating and evaporating the sourcematerial into a vapor. The container includes a heating zone which isdirectly heated by the heater and which is in contact with the sourcematerial over an effective contact area. Provided that the effectivecontact area is equal to S cm² and the volume of the source material isequal to V cm³, the invention requires V/S≦1 cm.

Since the container of the evaporation source is formed from aninsulator and closely surrounded by the heater, the evaporation sourcehas a very good thermal response and is adapted for the control of theevaporation rate. Satisfying V/S≦1 cm wherein the effective contact areais equal to S cm² and the volume of the source material is equal to Vcm³ ensures evaporation at a high rate and a high precision andtherefore, the deposition of a thin film of uniform thickness anduniform composition.

The heating zone of the container which is directly heated by the heaterincludes the inside surface portion of the container corresponding tothe surrounding heater and an additional inside surface portion of thecontainer which extends therefrom a distance of about 3 mm, specificallya distance of less than the thickness of the container wall. Providedthat the effective contact area of the heating zone with the sourcematerial is equal to S cm² and the volume of the source material isequal to V cm³, the invention requires that V/S is up to 1 cm,preferably from about 0.2 to 0.9 cm, and more preferably from about 0.4to 0.8 cm. The effective contact area of the heating zone in contactwith the source material is the inside surface area of the container(including its bottom, side wall and optional raised portion andsurrounded by the heater) that is in contact with the source material.The volume of the source material V (cm³) is preferably at least 10 cm³,and more preferably 20 to 100 cm³. The maximum volume of the sourcematerial that can be admitted into the container should preferably besmaller than the interior volume of the container. The reason is that aslight drop of temperature occurs at the opening of the containerbecause of radiation due to evaporation and heat dissipation to a holderor similar member, and an extra heating zone is thus necessary forcompensating for such a temperature drop.

The container of the evaporation source is formed from an insulatorhaving a thermal conductivity of at least 50 W/m·k, preferably at least75 W/m·k, more preferably at least 100 W/m·k, and most preferably atleast 125 W/m·k. The upper limit of thermal conductivity is not criticalalthough it is usually about 300 W/m·k. Exemplary insulators having sucha thermal conductivity include aluminum nitride, boron nitride, andpyrolytic boron nitride (PBN), with the PBN being preferred. Pyrolyticboron nitride may be formed by CVD or other processes. Whether or notboron nitride is pyrolytic can be determined by an analysis of crystalstructure by x-ray diffractometry (XRD). Specifically, in the case ofpyrolytic boron nitride, a peak of the [002] face is mainly detected inXRD among hexagonal BN crystal orientations, and its intensity isoutstandingly greater than those of other faces such as [100], [101],[102], and [001], which is different from the x-ray peaks of otherhexagonal BN. Pyrolytic boron nitride has the composition of BN, but maysomewhat deviate from the stoichiometry.

Around the container of the evaporation source is closely arranged theresistance heater. The heater is not critical as long as it can beformed in close contact with the outer periphery of the evaporationsource container. For example, graphite is directly deposited on thecontainer to form a thin film thereof, or a film heater comprisingpolyimide and stainless steel foils is attached to the container. Ofthese, the graphite thin film directly deposited on the container ispreferred because of a good thermal response. The graphite used hereinmay be pyrolytic graphite (PG). The pyrolytic graphite can be formed byCVD or other processes. The pyrolytic graphite by CVD is more firmly andclosely joined to the container.

Typically the container has a bottom and a side wall which togetherdefine the heating zone. That is, the heater closely surrounding thecontainer is preferably formed at least on the side wall and bottom ofthe container to define the heating zone thereat. The container havingthe heating zone along its side wall and bottom enables rapid accurateevaporation in good response. In one preferred embodiment, the containerfurther has a raised portion extending from the bottom, and the heatingzone is also associated with the raised portion. More particularly, thecontainer has a hollow raised portion vertically extending from thebottom, especially at its center, and the heater is also formed in closecontact with the outside surface of the raised portion of the container.Typically, the container is of cylindrical shape having an open top anda closed bottom, and preferably having an annular flange or collarradially extending from the top. The heater on the side wall of thecylindrical container may be formed from immediately below the flange,or from the transition between the flange and the side wall, or from theflange to the side wall. By extending the heater on the side wall toimmediately below the flange or along the flange, it becomes possible toprevent the vapor from depositing on the flange and prevent thetemperature lowering phenomenon that otherwise occurs at the opening ofthe container because of radiation and heat dissipation to a holder orsimilar member.

The heat release value of the heater may be determined as appropriatedepending on the dimensions of the evaporation source, the type ofsource material, the area to be covered, etc. Usually, an input power ofabout 50 to 500 W is applied to the heater while the heater has aresistance per unit length of about 3 to 3,000 Ω.

It is recommended that the heater is further surrounded by a layer of aninsulator having a thermal conductivity of at least 50 W/m·k. Theprovision of the outside insulator layer ensures insulation and improvesthe inward heat transfer from the heater. The preferred insulators usedherein are the same as the above-mentioned insulators, especially PBN.

The dimensions of the evaporation source may be determined asappropriate depending on the scale of the overall system and the size ofa substrate on which a material is to be evaporated. When the containeris a cylindrical crucible with an annular flange or collar, its innerdiameter is usually about 5 to 100 mm and preferably about 20 to 100 mm.The outer diameter of the container is approximately equal to the innerdiameter plus the flange width (both sides) of 30 to 70 mm, especially40 to 60 mm. The depth of the cylindrical crucible is about 5 to 200 mmand preferably about 5 to 100 mm. The wall gage is usually about 0.3 to5.0 mm and preferably about 0.5 to 2 mm when the breakage resistance andheat transfer of the container are taken into account. The depth of thecontainer to which the source material can be admitted is closelycorrelated to the area of the opening. When the heating zone is definedby the side wall and bottom of the container, the opening area ispreferably at least about 10 cm² and more preferably about 20 to 80 cm²and the material-receiving depth of the container is preferably up toabout 20 mm and more preferably about 5 to 15 mm. In the preferredembodiment wherein the container has a raised portion extending from thebottom and externally lined with the heater, the opening area ispreferably at least about 5 cm² and more preferably about 8 to 40 cm²and the material-receiving depth of the container is preferably up toabout 200 mm and more preferably about 20 to 100 mm.

The maximum volume of source material that can be admitted into thecontainer is usually at least 10 cm³ and preferably about 20 to 100 cm³,although it varies with the shape of the container.

In the evaporation source of the invention, the heater is preferablycapable of evaporating the source material at a maximum evaporation rateof at least 150 μg/sec, more preferably at least 200 μg/sec and mostpreferably 250 to 500 μg/sec.

Several requirements are imposed on the evaporation source. It isrequired that (1) precise and steady temperature control is possible,(2) the deposition rate is high enough to accommodate mass production,(3) the container can receive a sufficient amount of source material tocover large size substrates, (4) the container is not chemicallyreactive with constituent materials of organic EL devices, and (5) asource material can be evaporated to a desired vapor state and diffusedin a stable manner. Of these, requirements (1) to (4) are alreadydiscussed. Preferably the evaporation source of a shape satisfying theserequirements is used. One preferred evaporation source is a Knudsencell. The Knudsen cell is a cell having a predetermined opening as avapor effusion port. Provided that the opening has a diameter d and athickness t, the distribution of vapor density exiting from the effusionport takes the shape of a candle flame and is approximated by the shapeof cos^(n) θ. The value of n is given by approximating a vapor density mat an arbitrary position relative to the vapor density m₀ at the centerby m=m₀·cos^(n) θ. Therefore, a vapor density m₀ appears at a verticaldistance L₀ from the center of the opening, a vapor density m appears ata position spaced a distance L from the center of the opening at anangle θ, and the value of n obtained by approximating the vapor densitym to be m=m₀·(L₀/L)²·cos^(n) θ. As d/t decreases, the n value increasesand the shape of a flame becomes acute. In case of t=0 at the extremity,n=1 resulting in a spherical distribution standing on the opening. Thiscorresponds to the evaporation from an open liquid surface and is knownas Langmuir evaporation. Preferably n has a value of up to 6, andespecially from 3 to 5.

Also preferably, the evaporation source is disposed relative to thesubstrate such that the angle between a line connecting the center ofthe opening of the evaporation source and the center of the substrateand the surface of the substrate is from 20° to 60°, preferably from 30°to 60°. By effecting evaporation from an oblique direction with respectto the substrate surface, step coverage is improved. Then an organiclayer is formed so as to cover any contaminant or foreign particle on asubstrate (or a hole injecting electrode or in the case of a reverselystacked structure, an electron injecting electrode), avoiding theoccurrence of current leakage. Additionally, the electron injectingelectrode such as a metal thin film or the hole injecting electrode suchas ITO is improved in film physical properties. If the angle between theline connecting the centers of the evaporation source opening and thesubstrate and the substrate surface is more than 60°, step coverage isaggravated. Another problem associated with an angle of more than 60° isthat as the substrate size increases, the distance between theevaporation source and the substrate must be increased in order toachieve a uniform film thickness distribution, which results in a lowerdeposition rate. If the angle is less than 20°, the film thicknessdistribution becomes non-uniform and the distance between the substrateand the evaporation source must be increased along an extension line ofthat angle in order to achieve a uniform film thickness distribution,which undesirably requires an apparatus of greater size beyond thepractically acceptable size.

Understandably, the substrate may be inclined relative to a horizontalplane. In this case, the angular relationship between the substrate andthe evaporation source is maintained the same as above. The angle of thesubstrate relative to the horizontal plane is usually from 0° to 60°although it is not particularly limited insofar as the angle between thesubstrate and the evaporation source falls within the above-definedrange. As long as the above requirements are met, the substrate may bestationary or rotating. Rotation of the substrate further improves stepcoverage and allows a film of uniform quality and thickness distributionto deposit.

A too close distance between the substrate and the evaporation sourcetends to obstruct uniform evaporation over the entire surface of thesubstrate whereas a too long distance tends to lower the depositionrate. Then, the distance between the substrate and the evaporationsource, that is, the minimum distance between the horizontal plane wherethe center of the opening of the evaporation source is located and thehorizontal plane where the center of the substrate is located ispreferably 1.0 to 3.0 times, more preferably 1.5 to 2.5 times thedistance between the center and the edge of the substrate. A pluralityof evaporation sources may be arranged concentrically about thesubstrate center, all within the above range of substrate-to-sourcedistance. In this case, co-evaporation may be effected.

The improved evaporation source allows for efficient utilization of thesource material, particularly when a sublimable material is used, thepercent utilization of the material (the quantity of depositablematerial divided by the quantity of charged material) can be at least85%, more preferably at least 90%, and most preferably at least 95%. Anexpensive organic material can be efficiently utilized withoutsubstantial waste, contributing to a manufacture cost reduction.

Preferably the evaporation source has a gas cooling system which isshielded from the atmosphere of the evaporating chamber. The gas coolingsystem provides an improved cooling effect, with which the thermalresponse rate is increased. This embodiment is effective especially forthe low-temperature evaporation of organic materials. One exemplarycooling system includes a jacket disposed outside the evaporation sourcecontainer with the heater integrally formed therewith so as to define aspace between the container and the jacket wherein a gas coolant iscirculated through the space. The jacket is tightly joined to thecontainer using O-rings or other sealing members. The cooling system issealed in this way in order to maintain the vacuum within theevaporating chamber.

The gas coolant is preferably a gas having a predetermined thermalconductivity, least reactivity with the cell, and ease of handling.Specifically, the coolant gas preferably has a thermal conductivity ofat least 0.015 W/m·k, more preferably at least 0.025 W/m·k, and mostpreferably at least 0.15 W/m·k. Examples of the coolant gas includeinert gases such as He, Ne and Ar and least reactive gases such asnitrogen (N₂). Helium and nitrogen gases are preferred among others. Amixture of two or more of these gases is also useful while the mixingratio is arbitrary.

The flow rate of the gas coolant varies with the heat capacity of theevaporation source, the heat release value of the heater, etc. althoughit is usually about 50 to 5,000 SCCM. The manner of controlling the flowrate is not critical although a mass flow control mode is preferable.The direction of gas coolant flow is typically upward.

Now, a more illustrative construction of the evaporation sourceaccording to the invention is described. Referring to FIG. 1, there isillustrated an exemplary evaporation source 20 according to oneembodiment of the invention. FIG. 1A is a plan view and FIG. 1B is across-sectional view taken along line A-A′ in FIG. 1A. The evaporationsource 20 includes a cylindrical container 25 having a side wall 22 anda bottom 23 as well as an annular flange 21. A heater 24 closelysurrounds the container 25. Specifically, the heater 24 is formed on theoutside surface of the container 25 to a predetermined pattern (e.g.,helical pattern) using pyrolytic graphite. An insulating layer 26 coversthe heater 24. Specifically, the insulating layer 26 is formed on theheater 24 using the same material as the container 25. In theillustrated embodiment, the heater 24 further extends along the flange21 for preventing the vapor from depositing on the flange 21.

As depicted in FIG. 1, the container 20 has an inner diameter D, anouter diameter E and a depth F, while G designates the depth or heightto which the source material is admitted.

Referring to FIG. 2, there is illustrated an exemplary evaporationsource 30 according to another embodiment of the invention. FIG. 2A is aplan view and FIG. 2B is a cross-sectional view taken along line B-B′ inFIG. 2A. The evaporation source 30 includes a generally cylindricalcontainer 35 having a side wall 32 and a bottom 33 as well as an annularflange 31. The container 35 further has a raised portion 37 in the formof a concentric inner cylinder. A heater 34 closely surrounds thecontainer 35. Specifically, the heater 34 is formed on the outsidesurface of the container 35 to a predetermined pattern using pyrolyticgraphite. An insulating layer 36 covers the heater 34. Specifically, theinsulating layer 36 is formed on the heater 34 using the same materialas the container 35. In this embodiment, the heater 34 and theinsulating layer 36 also extend along the outside surface of the innercylinder 37. Herein, the inside surface of the container is to receivethe source material or define the heating zone, and the opposite surfaceof the container is designated the outside surface.

Also in FIG. 2, the container 30 has an inner diameter D, an outerdiameter E and a depth F, while G designates the depth or height of thesource material charge.

The second aspect of the invention is an apparatus for preparing anorganic EL device, comprising the improved evaporation source, asubstrate on which the organic EL device is to be formed, a means fordetecting the rate of evaporation of the source material on thesubstrate, and a means for controlling the evaporation source inaccordance with information from the detecting means.

The means for detecting the rate of evaporation of the source materialis not critical insofar as it can detect a time series change of theamount of source material depositing on the substrate. A choice may bemade among well-known evaporation rate detectors. Illustratively, adetector is combined with an oscillator (e.g., of quartz) such that asource material deposited on the oscillator is detectable as a change ofnatural oscillation of the oscillator.

A signal or information representative of the evaporation rate detectedby the detector is delivered to the control means. Based on the signalor information representative of the evaporation rate from the detector,the control means controls the evaporation source so as to maintain theevaporation rate constant. To this end, the control means directlycontrols the electric current or power applied to the heater of theevaporation source. If a temperature control system is built in theexisting evaporation apparatus or available as an off-the-shelf product,the control means cooperates with the temperature control system so asto achieve the predetermined temperature.

The desired range within which the evaporation rate is to be controlledvaries with the type of source material, etc. For organic materials oforganic EL devices, the evaporation rate is desirably in the range of0.05 to 0.6 nm/sec, more desirably 0.1 to 0.5 nm/sec, most desirably 0.3to 0.5 nm/sec, as measured at a height straight above the opening of theevaporation source and corresponding to the position of the substrate.It is noted that if the source material is a mixture of a host materialand a dopant, the evaporation rate of the host material is usually inthe above range while the evaporation rate of the dopant is in a rangeof 0.1 to 10% of the evaporation rate of the host material.

The source material to be evaporated is not critical insofar as it is aconstituent material of organic EL devices. It is preferred that theevaporating temperature of the source material that is equal to thetemperature of the evaporation source during evaporation is up to 800°C., preferably up to 500° C. The lower limit is not critical although itis usually about 150° C. Of these source materials, organic materialsused in light emitting layers to be described later are especiallypreferred. This is because such organic materials are evaporated atrelatively low temperatures and a slight change of the dose or mixingamount of a dopant has a significant influence on the devicecharacteristics. Therefore, the present invention is effectiveparticularly when accurate control of evaporating amounts is requisiteas in the case of multi-source evaporation of organic materials.

The main control means is not particularly limited in constructioninsofar as it can analyze the information delivered from the evaporationrate detecting means and carry out heater control in accordancetherewith. The control means is usually a general-purpose microprocessor(MPU) which is combined with a storage medium (ROM, RAM, etc.) bearing acontrol algorithm. Any of microprocessors including CISC, RISC, and DSPmay be used as the control means. Besides, the control means may beconstructed by ASIC, a combination of logic circuits by common ICs, oran analog arithmetic circuit using an operational amplifier.

The substrate used herein is not critical as long as an organic ELdevice can be stacked thereon. Where emitted light exits from thesubstrate side, transparent or translucent materials such as glass,quartz and resins are employed. The substrate may be provided with acolor filter film, a fluorescent material-containing color conversionfilm or a dielectric reflecting film for controlling the color of lightemission. Where emitted light exits from the side opposite to thesubstrate, the substrate may be either transparent or opaque. Ceramicsmay be employed as the opaque substrates.

The size of the substrate is not critical. Preferably, the substrate hasa maximum length of about 200 to about 700 mm, especially about 400 toabout 700 mm, which is a diagonal length for a typical rectangularsubstrate. Although a maximum length of less than 200 mm is not aproblem, the advantage of the invention that a uniform film thicknessdistribution is accomplished even on substrates of larger size becomesoutstanding with a maximum length of more than 200 mm. However, asubstrate size in excess of 700 mm would give rise to problems includinga larger size of film forming apparatus, low deposition efficiency, anddifficulty of film thickness control. The apparatus of the invention iscapable of precision control of the evaporation rates of sourcematerials located at different radial positions, thereby controlling thefilm thickness. More particularly, by arranging a plurality ofevaporation sources at concentric circles having different radii andaccurately controlling the evaporation rates of these evaporationsources to the predetermined values, the thickness distribution of afilm on a large size substrate is improved.

Referring to FIGS. 3 to 6, the construction and operation of theapparatus of the invention are illustrated in more detail.

FIG. 3 is a schematic view showing one exemplary construction of theapparatus for producing organic EL devices according to the invention.The apparatus includes a substrate 1 (only a part thereof is shown), anevaporation source 2 which may be either one of FIGS. 1 and 2, anevaporation rate detector 5, a control unit 6, and a heater power supply7 coupled with the control unit 6. The evaporation source includes acontainer 2 and a heater 3 formed or mounted closely on the container 2and connected to the power supply 7. A temperature sensor 4 isassociated with the evaporation source 2 for detecting the temperatureof the source. The detector 5 and the sensor 4 are coupled to thecontrol unit 6 so that the control unit 6 receives a signalrepresentative of the evaporation rate detected by the detector 5 and asignal representative of the source temperature detected by the sensor4. The control unit 6 includes a temperature control unit 6 a whichfunctions to control the power supply 7 on the basis of the temperaturedata detected by the sensor 4, so that the temperature of theevaporation source may become the predetermined value (or presenttemperature). The power supply 7 supplies a controlled electric currentor power to the heater 3 to generate the necessary heat.

In the apparatus thus constructed, the temperature of the evaporationsource is controlled at the start of evaporation as shown in FIG. 4.More particularly, the control unit 6, specifically the temperaturecontrol unit 6 a controls the power supply 7 such that the evaporationsource 2 is heated until the predetermined value (or preset temperature)is reached. The temperature T of the evaporation source rises as shownby the left curve in the graph of FIG. 4.

When the temperature T of the evaporation source reaches stable point Aof the predetermined value, the control unit 6 changes over its controlmode from the temperature control to the evaporation rate control.Switching from the control by the temperature control unit 6 a, thecontrol unit 6 now directly controls the power supply 7 so as to providethe predetermined evaporation rate. The evaporation rate R rises furtherand stabilizes at the predetermined value. In this way, the sourcetemperature is controlled before the start of evaporation and once theevaporating temperature reaches the predetermined value, the evaporationrate is controlled. As a consequence, the rise time of evaporation ratebecomes short, the hunting phenomenon and fluctuation or variation ofdeposition rate are suppressed, and stable evaporation takes place. Thethin film thus deposited becomes uniform in quality and thickness.

In the above embodiment, dual shutters are provided between theevaporation source and the substrate, one straight above the evaporationsource and one immediately below the substrate. The shutter on theevaporation source side is opened at the start of measurement of theevaporation rate, and the shutter on the substrate side is opened at thestart of deposition after the evaporation rate is stabilized. Then thesource material starts to deposit on the substrate after the evaporationrate is stabilized. The invention thus achieves film deposition andcontrol at high precision.

In the graph of FIG. 4, the temperature T of the evaporation source isincreased stepwise. Stepwise heating is a kind of preheating also knownas soaking which is generally intended for drying and is effectiveherein for the evaporation of organic materials. By carrying out initialheating under temperature control, effective soaking is achieved.

The temperature control unit 6 a may be constructed such that it mayoperate either as a part of the control unit 6 (in hardware or software)or independently of the control unit 6. When a conventional temperaturecontrol unit is used in its original state, it may be controlled as aunit separate and independent from the control unit 6. The temperaturecontrol unit 6 a is not particularly limited insofar as it can carry outappropriate temperature control. For example, a hardware (analog ordigital circuit) to which the proportional integral differential (PID)control mode is applied or a control algorithm having the mode extendedmay be used.

The temperature sensor may be selected from well-known temperaturesensors. A sensor capable of precise detection at the evaporatingtemperature of source materials is recommended. The range of temperatureto be measured is usually from about 20° C. to about 800° C. although itvaries with the type of source material. The temperature sensors usefulin such application include thermocouples, platinum thermometers andthermistors.

FIG. 5 illustrates a second example of the apparatus of the invention.In this example, the evaporation rate is controlled at the predeterminedvalue by controlling the temperature throughout the process. The controlof the evaporation rate based on consistent temperature controleliminates the hunting phenomenon of the deposition rate which can occurin the control mode shown in FIG. 4. More illustratively, in the initialheating stage, temperature control is carried out as in FIG. 4 until thepredetermined temperature is reached. In this stage too, soaking asshown in the graph is carried out, if desired.

Next, the control unit 6 carries out further heating through thetemperature control unit 6 a. By monitoring the deposition rate atintervals, the temperature control unit 6 a (for setting the heatingtemperature) is controlled so that the deposition rate may become thepredetermined (or preset) value. As in the first example, dual shuttersprovided between the evaporation source and the substrate are similarlyoperated. By controlling the evaporation rate by way of temperaturecontrol even after the temperature of the evaporation source 2 reachesthe predetermined value, the hunting phenomenon and fluctuation orvariation associated with the rise of the deposition rate can besuppressed and minimized.

In the control mode of FIG. 5 wherein temperature control is carriedout, after the predetermined deposition rate is reached, so as to keepthe evaporation rate constant, it is sometimes difficult to keep thedeposition rate constant. To avoid the instability of control, a controlmode as shown in FIG. 6 is effective.

FIG. 6 illustrates a third example of the apparatus of the invention. Inthis example, temperature control is carried out until the predeterminedevaporation rate is reached, while monitoring the evaporation rate R asin the embodiment of FIG. 5. Thereafter, the control mode is changedover to the mode of directly controlling the heater power supply 7 inaccordance with the evaporation rate as in the embodiment of FIG. 4.That is, the control unit 6 carries out temperature control until theevaporation rate R reaches the predetermined value, as in the example ofFIG. 4, and after the evaporation rate R reaches the predetermined valueB, the control unit 6 carries out control to maintain the evaporationrate R at the predetermined value B, by controlling the heater powersupply 7 directly and not by way of the temperature control unit 6 a,for controlling the electric current or power to the heater 3. Thisexample is effective for restraining the temperature T from rising afterthe evaporation rate is stabilized, which can occur in the control modeof FIG. 4.

Although several control modes using the apparatus of the invention havebeen described, an optimum one may be selected from them by taking intoaccount the type of source material, the size of the apparatus and otherfactors.

The apparatus of the invention allows for heating control with a quicktemperature rise, adequate soaking, and stable control of evaporationrate and thus enables to form an evaporated thin film of uniform qualityand thickness which is difficult to form by prior art evaporationprocesses. The invention is thus effective in forming a functional thinfilm by evaporating an organic material, especially a light emittinglayer of organic EL devices. The invention also enables to accuratelyadjust and maintain the mixing quantities of two or more organicmaterials or the doping quantity in two or multi-source evaporation.Organic EL devices having consistent luminous performance can beprepared in a mass-scale manufacture process.

Thin films of organic EL devices which can be formed by the apparatus ofthe invention include a hole injecting and transporting layer, lightemitting layer, and electron injecting and transporting layer. Theinvention is also effective for improving film physical propertiesbetween an electrode and an organic layer as previously mentioned.Therefore, the invention is continuously applicable in forming a holeinjecting electrode or electron injecting electrode.

The organic layers which can be formed according to the invention areillustrated below.

The light emitting layer contains a fluorescent material that is acompound having a light emitting capability. The fluorescent materialmay be at least one member selected from compounds as disclosed, forexample, in JP-A 63-264692, such as quinacridone, rubrene, and styryldyes. Also, quinoline derivatives such as metal complex dyes having8-quinolinol or a derivative thereof as the ligand such astris(8-quinolinolato)aluminum are included as well astetraphenylbutadiene, anthracene, perylene, coronene, and12-phthaloperinone derivatives. Further useful are the phenylanthracenederivatives described in JP-A 8-12600 and the tetraarylethenederivatives described in JP-A 8-12969.

It is preferred to use the fluorescent material in combination with ahost material capable of light emission by itself, that is, to use thefluorescent material as a dopant. In this embodiment, the content of thefluorescent material in the light emitting layer is preferably 0.01 to10% by weight, especially 0.1 to 5% by weight. By using the fluorescentmaterial in combination with the host material, the light emissionwavelength of the host material can be altered, allowing light emissionto be shifted to a longer wavelength and improving the luminous efficacyand stability of the device.

As the host material, quinolinolato complexes are preferable, withaluminum complexes having 8-quinolinol or a derivative thereof as theligand being more preferable. These aluminum complexes are disclosed inJP-A 63-264692, 3-255190, 5-70733, 5-258859 and 6-215874.

Illustrative examples include tris(8-quinolinolato)aluminum,bis(8-quinolinolato)magnesium, bis(benzo{f}-8-quinolinolato)zinc,bis(2-methyl-8-quinolinolato)aluminum oxide,tris(8-quinolinolato)indium, tris(5-methyl-8-quinolinolato)aluminum,8-quinolinolatolithium, tris(5-chloro-8-quinolinolato)gallium,bis(5-chloro-8-quinolinolato)calcium,5,7-dichloro-8-quinolinolato-aluminum,tris(5,7-dibromo-8-hydroxyquinolinolato)aluminum, andpoly[zinc(II)-bis(8-hydroxy-5-quinolinyl)methane].

Also useful are aluminum complexes having another ligand in addition to8-quinolinol or a derivative thereof. Examples includebis(2-methyl-8-quinolinolato)(phenolato)aluminum(III),bis(2-methyl-8-quinolinolato)(ortho-cresolato)aluminum(III),bis(2-methyl-8-quinolinolato)(meta-cresolato)aluminum(III),bis(2-methyl-8-quinolinolato)(para-cresolato)aluminum(III),bis(2-methyl-8-quinolinolato)(ortho-phenylphenolato)aluminum(III),bis(2-methyl-8-quinolinolato)(meta-phenylphenolato)aluminum(III),bis(2-methyl-8-quinolinolato)(para-phenylphenolato)aluminum(III),bis(2-methyl-8-quinolinolato)(2,3-dimethylphenolato)aluminum(III),bis(2-methyl-8-quinolinolato)(2,6-dimethylphenolato)aluminum(III),bis(2-methyl-8-quinolinolato)(3,4-dimethylphenolato)aluminum(III),bis(2-methyl-8-quinolinolato)(3,5-dimethylphenolato)aluminum(III),bis(2-methyl-8-quinolinolato)(3,5-di-tert-butylphenolato)aluminum(III),bis(2-methyl-8-quinolinolato)(2,6-diphenyl-phenolato)aluminum(III),bis(2-methyl-8-quinolinolato)(2,4,6-triphenylphenolato)aluminum(III),bis(2-methyl-8-quinolinolato)(2,3,6-trimethylphenolato)aluminum(III),bis(2-methyl-8-quinolinolato)(2,3,5,6-tetramethylphenolato)aluminum(III),bis(2-methyl-8-quinolinolato)(1-naphtholato)aluminum(III),bis(2-methyl-8-quinolinolato)(2-naphtholato)aluminum(III),bis(2,4-dimethyl-8-quinolinolato)(ortho-phenylphenolato)aluminum(III),bis(2,4-dimethyl-8-quinolinolato)(para-phenylphenolato)aluminum(III),bis(2,4-dimethyl-8-quinolinolato)(meta-phenylphenolato)aluminum(III),bis(2,4-dimethyl-8-quinolinolato)(3,5-dimethylphenolato)aluminum(III),bis(2,4-dimethyl-8-quinolinolato)(3,5-di-tert-butylphenolato)aluminum(III),bis(2-methyl-4-ethyl-8-quinolinolato)(para-cresolato)aluminum(III),bis(2-methyl-4-methoxy-8-quinolinolato)(para-phenylphenolato)aluminum(III),bis(2-methyl-5-cyano-8-quinolinolato)(ortho-cresolato)aluminum(III), andbis(2-methyl-6-trifluoromethyl-8-quinolinolato)(2-naphtholato)aluminum(III).

Also acceptable arebis(2-methyl-8-quinolinolato)aluminum(III)-μ-oxo-bis(2-methyl-8-quinolinolato)aluminum(III),bis(2,4-dimethyl-8-quinolinolato)aluminum(III)-μ-oxo-bis(2,4-dimethyl-8-quinolinolato)aluminum(III),bis(4-ethyl-2-methyl-8-quinolinolato)aluminum(III)-μ-oxo-bis(4-ethyl-2-methyl-8-quinolinolato)aluminum(III),bis(2-methyl-4-methoxyquinolinolato)aluminum(III)-μ-oxo-bis(2-methyl-4-methoxyquinolinolato)aluminum(III),bis(5-cyano-2-methyl-8-quinolinolato)aluminum(III)-μ-oxo-bis(5-cyano-2-methyl-8-quinolinolato)aluminum(III),andbis(2-methyl-5-trifluoromethyl-8-quinolinolato)aluminum(III)-μ-oxo-bis(2-methyl-5-trifluoromethyl-8-quinolinolato)aluminum(III).

Other useful host materials are the phenylanthracene derivativesdescribed in JP-A 8-12600 and the tetraarylethene derivatives describedin JP-A 8-12969.

The light emitting layer may also serve as the electron injecting andtransporting layer. In this case, tris(8-quinolinolato)aluminum etc. arepreferably used.

The electron injecting and transporting compound is preferably selectedfrom quinoline derivatives and metal complexes having 8-quinolinol or aderivative thereof as a ligand, especially tris(8-quinolinolato)aluminum(Alq3). The aforementioned phenylanthracene derivatives andtetraarylethene derivatives are also preferable.

The compound for the hole injecting and transporting layer is preferablyselected from amine derivatives having strong fluorescence, for example,triphenyldiamine derivatives, styrylamine derivatives and aminederivatives having an aromatic fused ring.

The electron injecting electrode is preferably formed from materialshaving a low work function, for example, metal elements such as K, Li,Na, Mg, La, Ce, Ca, Sr, Ba, Al, Ag, In, Sn, Zn, and Zr, and binary orternary alloys made of two or three such metal elements for stabilityimprovement. Exemplary alloys are Ag-Mg (Ag: 0.1 to 50 at %), Al—Li (Li:0.01 to 14 at %), In—Mg (Mg: 50 to 80 at %), and Al—Ca (Ca: 0.01 to 20at %). It is understood that the electron injecting electrode can alsobe formed by evaporation or sputtering.

The electron injecting electrode thin film may have a sufficientthickness for electron injection, for example, a thickness of at least0.1 nm, preferably at least 1 nm. Although the upper limit is notcritical, the electrode thickness is typically about 1 to about 500 nm.On the electron injecting electrode, a protective electrode may beprovided, if desired. A protective layer may be formed using metalmaterials, inorganic materials such as SiOx, and organic materials suchas Teflon.

During evaporation, an appropriate pressure is 1×10⁻⁸ to 1×10⁻⁵ Torr andthe heating temperature of the evaporation source is about 100° C. toabout 1,400° C. for metal materials and about 100° C. to about 500° C.for organic materials.

The organic EL light-emitting device manufactured by the method of theinvention has a hole injecting electrode on a substrate and an electroninjecting electrode thereon. At least a hole transporting layer, a lightemitting layer and an electron injecting and transporting layer aredisposed between the electrodes. The device further has a protectiveelectrode as the uppermost layer. Of these layers, the hole transportinglayer, electron transporting layer, and protective electrode are omittedas the case may be.

A transparent or translucent electrode is preferred as the holeinjecting electrode because a structure allowing emitted light to exitfrom the substrate side is typical. Useful materials for transparentelectrodes include tin-doped indium oxide (ITO), zinc-doped indium oxide(IZO), zinc oxide (ZnO), tin oxide (SnO₂), and indium oxide (In₂O₃),with ITO and IZO being preferred. The ITO usually contains In₂O₃ and SnOin stoichiometry although the oxygen content may deviate somewhattherefrom.

The hole injecting electrode preferably has a transmittance of at least50%, more preferably at least 60%, further preferably at least 80%, andespecially at least 90% for each light emission in a luminous wavelengthband, typically of 350 to 800 nm. Since the emitted light exits thedevice through the hole injecting electrode, the hole injectingelectrode with a low transmittance causes the light emission toattenuate, failing to provide a necessary luminance as the lightemitting device. Where the emitted light is taken out of the devicesolely from one side, it suffices that the electrode on the take-outside has a transmittance of at least 50%.

The hole injecting electrode should have a sufficient thickness for holeinjection and is preferably about 50 to about 500 nm thick, especiallyabout 50 to 300 nm thick. Although no upper limit need be imposed on thethickness of the hole injecting electrode, too thick electrodes can peeloff. A too thin electrode is undesirable in film strength, holetransporting capability, and resistivity.

The hole injecting electrode can be formed by evaporation or otherprocesses although sputtering is preferable.

After the organic EL device layers are deposited, a protective film maybe formed using an inorganic material such as SiOx or an organicmaterial such as Teflon or chlorine-containing fluorocarbon polymer. Theprotective film may be transparent or opaque. Its thickness is typicallyabout 50 to 1,200 nm. The protective film may be formed by reactivesputtering as well as general sputtering, evaporation and PECVDprocesses.

On the substrate, a color filter film, a color conversion filmcontaining a fluorescent material, or a dielectric reflective layer maybe provided for controlling the color of emitted light.

The organic EL device of the invention is generally of the dc or pulsedrive type while it can be of the ac drive type. The applied voltage isgenerally about 2 to 30 volts.

EXAMPLE

Examples of the present invention are given below by way of illustrationand not by way of limitation.

Example 1

The evaporation source (Sample Nos. 1-3 and comparative Sample Nos. 4-7)had a container which was formed by CVD from PBN into a Knudsen cellshape configured as shown in Table 1. A helical heater was formed on thecontainer by depositing pyrolytic graphite by CVD. The container andheater were overcoated with PBN by CVD to form an insulative coating.

Sample No. 1 is of the shape of FIG. 1 having the heater on the bottomand side wall of the container. Sample Nos. 2 and 3 are of the shape ofFIG. 2 having the heater on the bottom, side wall and raised portion ofthe container. Sample Nos. 4 to 6 are of the shape of FIG. 1, but havingthe heater only on the side wall of the container.

Each of the thus obtained evaporation source samples (Knudsen cells) wascharged with a sublimable material Alq3 as the source material andheated to evaporate the source material into a vapor. The value of n,the effective contact area (S cm²), the volume of the source material (Vcm³), and V/S were determined. A maximum evaporation rate wasdetermined. A percent utilization of the source material (depositablesource material/source material charge) was also determined. The resultsare shown in Table 1.

TABLE 1 Heating Material zone Bottom Raised portion charge Inner Openingcontact Volume of Evaporation heater heater outer Material Sample depthdiameter area n area material V rate area diameter utilization No. (mm)(mm) (cm²) value S:(cm²) (cm³) V/S (μg/sec) (cm²) (mm) (%) 1 10 60 28.33 47.1 28.3 0.6 300 28.3 — 90 2 10 40 9.4 4 28.3 9.4 0.33 200  9.4 20 953 50 40 9.4 4 103.7 47.1 0.45 200  9.4 20 95 4 10 20 3.1 5 6.3 3.1* 0.49 100* — —  80* 5 50 20 3.1 5 34.6 15.7 0.45  100* — — 90 6 10 60 28.3 318.8 28.3 1.50* 150 — —  50* 7 50 60 28.3 3 122.5 141.4 1.15* 150 28.3 — 60* Sample Nos. 4 to 7 are comparative samples. *indicates outside thescope of the invention

As seen from Table 1, the evaporation source samples within the scope ofthe invention have a high evaporation rate and a high materialutilization and are thus advantageous in depositing organic ELmaterials.

Example 2

Using the evaporation source of Example 1, there was furnished anevaporation apparatus based on the control mode of the first embodiment(FIGS. 3 and 4). The apparatus was operated to deposit a layer of Alq3doped with coumarin on a substrate as a light emitting layer. Avariation of the doping quantity was determined. Evaporation was carriedout many times from the initial state that the container was fullycharged with the source material until the quantity of source materialwas reduced to the limit above which stable evaporation was possible.Determined from a change of evaporation rate was a volume ratio ofAlq3/coumarin, from which a variation of the coumarin doping quantitywas calculated. The apparatus was controlled so as to provide a coumarindoping quantity of 1% by volume.

As a result, the apparatus using the comparative sample showed a dopingquantity variation of ±10%, and the apparatus using the inventive sampleshowed a doping quantity variation of ±5%.

The evaporation source of the invention is capable of containing a largeamount of source material, enables stable evaporation over a long periodof time, enables to adjust and maintain uniform the thickness andcomposition of a thin film, and allows for evaporation at relatively lowtemperatures or on a substrate with a relatively large surface area. Theevaporation source is useful in an apparatus and method for thepreparation of an organic EL device. In the event of multi-sourceevaporation, the organic EL device-preparing apparatus and method cancontrol at high precision the mixing ratio or doping amount.

Japanese Patent Application No. 10-256058 is incorporated herein byreference.

The invention has been described in detail with particular reference topreferred embodiments thereof, but it will be understood that variationsand modifications can be effected within the spirit and scope of theinvention.

What is claimed is:
 1. An evaporation source for evaporating a source material, the evaporation source comprising a container comprising an insulator and having an exterior surface and an interior surface, the interior surface defining a volume V to hold the source material and having a surface area S, with V/S≦1 cm; and a heater in direct contact with the exterior surface of the container, wherein the container further comprises a bottom including a raised portion; and a portion of the exterior surface and a portion of the heater are inside the raised portion.
 2. The evaporation source of claim 1, wherein the volume V defined by the interior surface of the container is at least 5 cm³.
 3. The evaporation source of claim 1, wherein the insulator has a thermal conductivity of at least 50 W/m·k.
 4. The evaporation source of claim 1, wherein the container further comprises a second layer, on the heater, comprising an insulator having a thermal conductivity of at least 50 W/m·k.
 5. The evaporation source of claim 4, wherein at least one of the insulator of the container and the insulator of the second layer comprises pyrolytic boron nitride; and the heater comprises carbon.
 6. An apparatus comprising the evaporation source of claim 1; a substrate and an evaporation rate detector above the evaporation source; and an evaporation source controller electrically connected to the evaporation rate detector and the evaporation source.
 7. A method of using an evaporation source, the method comprising placing a source material in the evaporation source of claim 1; actuating the heater to evaporate the source material; detecting a rate of evaporation of the source material; and controlling the evaporation source on the basis of the detected rate of evaporation.
 8. The method of claim 7, wherein the controlling of the evaporation source maintains the rate of evaporation constant.
 9. The method of claim 7, wherein the controlling of the evaporation source controls a temperature of the evaporation source.
 10. The method of claim 7, wherein the controlling of the evaporation source controls an electric current or power applied to the heater of the evaporation source.
 11. The method of claim 7, wherein the controlling of the evaporation source includes controlling a temperature of the evaporation source, and then maintaining the rate of evaporation constant.
 12. The evaporation source of claim 1, wherein the heater comprises a conductive strip deposited on the exterior surface of the container.
 13. The evaporation source of claim 12, wherein conductive strip comprises graphite. 